Room-Temperature Processed Nb₂O₅ as the Electron-Transporting Layer for Efficient Planar Perovskite Solar Cells

In this work, we demonstrate high-efficiency planar perovskite solar cells (PSCs), using room-temperature sputtered niobium oxide (Nb₂O₅) as the electron-transporting layer (ETL). Widely spread ETL-like TiO₂ often requires high-temperature (>450 °C) sintering, which is not desired for the fabrication of flexible devices. The amorphous Nb₂O₅ (labeled as a-Nb₂O₅) ETL, without any heat treatment, can give a best power conversion efficiency (PCE) of 17.1% for planar PSCs. Interestingly, the crystalline Nb₂O₅ (labeled as c-Nb₂O₅), with high-temperature (500 °C) annealing, results in a very similar PCE of 17.2%, indicating the great advantage of a-Nb₂O₅ in energy saving. We thus carried out a systematical investigation on the properties of the a-Nb₂O₅ film. The Hall effect measurements indicate both high mobility and conductivity of the a-Nb₂O₅ film. Kelvin probe force microscopy measurements define the Fermi levels of a-Nb₂O₅ and c-Nb₂O₅ as -4.31 and -4.02 eV, respectively, which allow efficient electron extraction at the Nb₂O₅/perovskite interface, regardless of the additional heat treatment on Nb₂O₅ film. Benefitting from the low-temperature process, we further demonstrated flexible PSCs based on a-Nb₂O₅, with a considerable PCE of 12.1%. The room-temperature processing and relatively high device performance of a-Nb₂O₅ suggest a great potential for its application in optoelectrical devices.